Aromatic hydrocarbons, particularly benzene, toluene, ethylbenzene and xylenes, are important commodity chemicals in the petrochemical industry. Currently, aromatics are most frequently produced from petroleum-based feedstocks by a variety of processes, including catalytic reforming and catalytic cracking. However, as the world supplies of petroleum feedstocks decrease, there is a growing need to find alternative sources of aromatic hydrocarbons.
One possible alternative source of aromatic hydrocarbons is methane, which is the major constituent of natural gas. World reserves of natural gas are constantly being upgraded and more natural gas is currently being discovered than oil. Because of the problems associated with transportation of large volumes of natural gas, most of the natural gas produced along with oil, particularly at remote places, is flared and wasted. Hence the conversion of alkanes contained in natural gas directly to higher hydrocarbons, such as aromatics, is an attractive method of upgrading natural gas, providing the attendant technical difficulties can be overcome.
A large majority of the processes currently proposed for converting methane to liquid hydrocarbons involve initial conversion of the methane to synthesis gas, a blend of H2 and CO. However, production of synthesis gas is capital and energy intensive and hence routes that do not require synthesis gas generation are preferred.
A number of alternative processes have been proposed for directly converting methane to higher hydrocarbons. One such process involves catalytic oxidative coupling of methane to olefins followed by the catalytic conversion of the olefins to liquid hydrocarbons, including aromatic hydrocarbons. For example, U.S. Pat. No. 5,336,825 discloses a two-step process for the oxidative conversion of methane to gasoline range hydrocarbons comprising aromatic hydrocarbons. In the first step, methane is converted to ethylene and minor amounts of C3 and C4 olefins in the presence of free oxygen using a rare earth metal promoted alkaline earth metal oxide catalyst at a temperature between 500° C. and 1000° C. The ethylene and higher olefins formed in the first step are then converted to gasoline range liquid hydrocarbons over an acidic solid catalyst containing a high silica pentasil zeolite.
However, oxidative coupling methods suffer from the problems that they involve highly exothermic and potentially hazardous methane combustion reactions and they generate large quantities of environmentally sensitive carbon oxides.
A potentially attractive route for upgrading methane directly into higher hydrocarbons, particularly ethylene, benzene and naphthalene, is dehydrocyclization or reductive coupling. This process typically involves contacting the methane with a catalyst comprising a metal, such as rhenium, tungsten or molybdenum, supported on a zeolite, such as ZSM-5, at high temperature, such as 600° C. to 1000° C. Frequently, the catalytically active species of the metal is the zero valent elemental form or a carbide or oxycarbide.
For example, U.S. Pat. No. 4,727,206 discloses a process for producing liquids rich in aromatic hydrocarbons by contacting methane at a temperature between 600° C. and 800° C. in the absence of oxygen with a catalyst composition comprising an aluminosilicate having a silica to alumina molar ratio of at least 5:1, said aluminosilicate being loaded with (i) gallium or a compound thereof and (ii) a metal or a compound thereof from Group VIIB of the Periodic Table.
In addition, U.S. Pat. No. 5,026,937 discloses a process for the aromatization of methane which comprises the steps of passing a feed stream, which comprises over 0.5 mole % hydrogen and 50 mole % methane, into a reaction zone having at least one bed of solid catalyst comprising ZSM-5, gallium and phosphorus-containing alumina at conversion conditions which include a temperature of 550° C. to 750° C., a pressure less than 10 atmospheres absolute (1000 kPaa) and a gas hourly space velocity of 400 to 7,500 hr−1.
Moreover, U.S. Pat. No. 5,936,135 discloses a low temperature, non-oxidative process for the conversion of a lower alkane, such as methane or ethane, to aromatic hydrocarbons. In this process, the lower alkane is mixed with a higher olefin or paraffin, such as propylene or butene, and the mixture is contacted with a pretreated bifunctional pentasil zeolite catalyst, such as GaZSM-5, at a temperature of 300° C. to 600° C., a gas hourly space velocity of 1000 to 100000 cm3g−1hr−1 and a pressure of 1 to 5 atmosphere (100 to 500 kPa). Pretreatment of the catalyst involves contacting the catalyst with a mixture of hydrogen and steam at a temperature 400° C. to 800° C., a pressure of 1 to 5 atmosphere (100 to 500 kPa) and a gas hourly space velocity of at least 500 cm3g−1hr−1 for a period of at least 0.5 hour and then contacting the catalyst with air or oxygen at a temperature of 400° C. to 800° C., a gas hourly space velocity of at least 200 cm3g−1hr−1 and a pressure of 1 to 5 atmosphere (100 to 500 kPa) for a period of at least 0.2 hour.
A particular difficulty in using natural gas as an aromatics source concerns the fact that many natural gas fields around the world contain large quantities, sometimes in excess of 50%, of carbon dioxide. Not only is carbon dioxide a target of increasing governmental regulation because of its potential contribution to global climate change, but also any process that requires separation and disposal of large quantities of carbon dioxide from natural gas is likely to be economically prohibitive. In fact, some natural gas fields have such high carbon dioxide levels as to be currently considered economically unrecoverable.
There is therefore a need for an improved process for converting methane to aromatic hydrocarbons, particularly where the methane is present in a natural gas stream containing large quantities of carbon dioxide.
U.S. Pat. Nos. 6,239,057 and 6,426,442 disclose a process for producing higher carbon number hydrocarbons, e.g., benzene, from low carbon number hydrocarbons, such as methane, by contacting the latter with a catalyst comprising a porous support, such as ZSM-5, which has dispersed thereon rhenium and a promoter metal such as iron, cobalt, vanadium, manganese, molybdenum, tungsten or a mixture thereof. After impregnation of the support with the rhenium and promoter metal, the catalyst is activated by treatment with hydrogen and/or methane at a temperature of about 100° C. to about 800° C. for a time of about 0.5 hr. to about 100 hr. The addition of CO or CO2 to the feed is said to increase the selectivity to benzene and the stability of the catalyst. The ratio CO and/or CO2 to methane can vary from about 0.001 to about 0.5 and preferably from about 0.01 to about 0.3.
Russian Patent No. 2,135,441 discloses a process for converting methane to heavier hydrocarbons, in which the methane is mixed with at least 5 wt % of a C3+ hydrocarbon, such as benzene, and then contacted in a multi-stage reactor system with a catalyst comprising metallic platinum having a degree of oxidation greater than zero at a methane partial pressure of at least 0.05 MPa and a temperature of at least 440° C. Hydrogen generated in the process may be contacted with oxides of carbon to generate additional methane that, after removal of the co-produced water, can be added to the methane feed. The products of the methane conversion are a C2-C4 gaseous phase and a C5+ liquid phase but, according the Examples, there is little (less than 5 wt %) or no net increase in aromatic rings as compared with the feed.
In our co-pending U.S. Patent Application Ser. No. 60/638,922, filed on Dec. 22, 2004, the entire contents of which are incorporated herein by reference, we have described a process for converting methane to higher hydrocarbons including aromatic hydrocarbons, the process comprising (a) contacting a feed containing methane with a dehydrocyclization catalyst under conditions effective to convert the methane to aromatic hydrocarbons and produce a first effluent stream comprising aromatic hydrocarbons and hydrogen, wherein the first effluent stream comprises at least 5 wt % more aromatic rings than said feed; and (b) reacting at least part of the hydrogen from the first effluent stream with an oxygen-containing species, particularly CO and/or CO2, to produce a second effluent stream having a reduced hydrogen content compared with the first effluent stream.
Further, in our co-pending U.S. Patent Application Ser. No. 60/638,605, filed Dec. 22, 2004, the entire contents of which are incorporated herein by reference, we have described a process for converting methane to alkylated aromatic hydrocarbons, the process comprising contacting a feed containing methane with a dehydrocyclization catalyst under conditions effective to convert said methane to aromatic hydrocarbons and produce a first effluent stream comprising aromatic hydrocarbons and hydrogen; and contacting at least a portion of said aromatic hydrocarbon from said first effluent stream with an alkylating agent under conditions effective to alkylate said aromatic hydrocarbon and produce an alkylated aromatic hydrocarbon having more alkyl side chains than said aromatic hydrocarbon prior to the alkylating.
Aromatic hydrocarbons, like all hydrocarbons, inherently contain deuterium and 13C in amounts that can vary according to the source of the carbon and hydrogen atoms in the molecule. Moreover, studies of isotope distributions have shown that the amounts of deuterium and 13C in naturally-occurring geologic methane are significantly different from the amounts of deuterium and 13C in naphtha and that the amount of 13C in naturally-occurring geologic CO2 is significantly different from the amounts of 13C in naturally-occurring geologic methane and in naphtha. Thus the present invention is based on the realization that the amounts of deuterium and 13C present in an aromatic hydrocarbon product can serve to differentiate between an aromatic hydrocarbon produced from naphtha, the same aromatic hydrocarbon produced by dehydrocyclization of naturally-occurring geologic methane alone and the same aromatic hydrocarbon produced by dehydrocyclization of naturally-occurring geologic methane and methane produced from CO2. 